1. Field of the Invention
The present invention relates to a congestion control method for a wireless communication system and a base station control apparatus, and more specifically, to a congestion control method for a wireless communication system and a base station control apparatus for performing congestion control in a mobile communication system where a plurality of mobile stations can simultaneously perform communication in a single sector, a single mobile station can receive signals from a plurality of sectors, and a code division multiple access (CDMA) communication is performed in each sector.
2. Description of the Related Art
The basis of mobile communication is that a mobile station such as an in-vehicle phone or a portable phone captures radio waves from a sector of a nearby base station and exchanges radio waves with the base station. A serviceable range or the reach of radio waves from one of antennas mounted on the base station is referred to as “a sector”. The base station has one or more sectors. A mobile communication service area is formed by providing sectors without leaving a space. With a multi-sector structure formed by providing base stations at regular spacings, a plurality of sectors of the base stations provided closely (not sparsely) establishes a wide service area, allowing a wireless terminal to communicate from any place.
A single sector has finite wireless communication resources (connections) that can be used simultaneously by a plurality of mobile stations. In a CDMA mobile communication system, code patterns that can be used to start communication are used in descending order of intensities of radio waves, and an identical wireless channel is used as far as the intensities of radio waves allows the call to be continued. If the intensities of received radio waves decreases because of a movement of the mobile station or a change in the environment, a soft hand-over to radio waves of another sector occurs while the communication state is being maintained.
If many mobile stations attempt to start unplanned communication newly in an identical sector because of an accident or an event, the wireless resources are congested to disable communication. Some methods have been developed as techniques to handle the problem of congestions in wireless resources caused by a temporary increase in communication traffic. For example, the base station can restrict communication with mobile stations when the number of remaining resources such as transmitting and receiving apparatuses decreases. In a CDMA mobile communication system, when the amount of interference power increases, communication of the mobile station with the base station is restricted. The communication restriction includes the following: the mobile stations are classified into general mobile stations and priority mobile stations and are allowed to register their positions or not, to originate a call or not, and so on in stages, in accordance with the remaining resources and the amount of interference power, for instance. To make efficient use of wireless resources, the transmission power of a congested base station is decreased to force a soft hand-over of a mobile station to a nearby base station (refer to Japanese Unexamined Patent Application Publications No. Hei-6-69860 and No. 2002-77042).